Add GITHUB_REF detection

[AustinShalit]

This is a workaround for actions/checkout#290. As described in that issue, the goal of the checkout action is to always run on the provided build hash even if additional tags have been pushed. This means if a user pushed 2 tags at the same time, 2 build artifacts will result. One workaround proposed is to force pull tags. This workaround breaks the promise of GitHub builds – 2 tag pushes now can result in 2 independent builds but 1 build artifact total.

This PR adds detection for the GITHUB_REF environment variable. If we detect the variable and are in release mode, we will trust that the variable is correct. Otherwise, we fallback onto the already proven git describe method.

You can see it in action here:

master: https://github.com/AustinShalit/OutlineViewer/runs/1616714453?check_suite_focus=true
"beta 5": https://github.com/AustinShalit/OutlineViewer/runs/1616715084?check_suite_focus=true
"beta 6": https://github.com/AustinShalit/OutlineViewer/runs/1616715071?check_suite_focus=true

[sciencewhiz]

GITHUB_REF will have the branch name, if the commit isn't tagged. Is that handled elsewhere, or does there need to be a check for that?

[AustinShalit]

That is handled with this check for "tags":
https://github.com/wpilibsuite/wpilib-version-plugin/pull/27/files#diff-17b10058baae9d050ef9962903d08b5defec34ef16f760002f5e2d6e81d26613R60